Refactoring of linopy solvers to object oriented architecture

[daniel-rdt]

In this PR the solvers.py file is refactored to an object-oriented architecture. For this, a Solver superclass is introduced, as well as subclasses for each of the implemented solvers. A given problem can now be solved from a problem file without a linopy model using a specified solver. To solve a problem, for each Solver subclass the method solve_problem_file() is implemented. The implementation is based on the following logic suggested by @FabianHofmann in #198 (comment):

class Solver:
    def __init__(self, **kwargs):
        ....

    def solve_problem(self):
        raise NotImplementedError

class SolverA(Solver):
    ....
    def solve_problem(self, problem_fn):
        # Implementation of the solve method for SolverA
        ...
        return Result(status, solution)

class SolverB(Solver):
    ....
    def solve_problem(self, problem_fn):
        # Implementation of the solve method for SolverB
        ...
        return Result(status, solution)
....

To solve a problem directly from a problem file, for example using linopy's Gurobi solver interface:

import linopy
from pathlib import Path

problem_file = Path("path/to/problem.lp")

gurobi = linopy.solvers.Gurobi()
gurobi.solve_problem(problem_fn=problem_file)

For direct execution using a linopy model, the method set_direct_model used that adds the model to the Solver instance:

....
solver = solver_class()
solver.set_direct_model(model=model)
result = solver.solve_problem()

Two unit tests (test_solver_classes_from_problem_file and test_solver_classes_direct) were also added to test the functionality of the solver classes from a problem file and directly from a model. @finozzifa, any feedback on this is welcome.

[FabianHofmann]

Great work @daniel-rdt and really efficient! I try to find time to review this asap. In case I am not able to @lkstrp has to jump in.

Perhaps you try solving the type and test coverage warnings. I have the feeling that sense and io_api should not be mandatory arguments for initialization of the solver. The sense is given in the lp and mps files. So the current implementation would lead to ambiguity. Could you look into this?

[daniel-rdt]

I've addressed the type check issues (except the type check for the pandas groupby which was addressed by PR #352).

I've also adjusted the architecture of the direct solving from a linopy model vs solving from a problem file. The solver instance is not initialized with io_api and sense anymore. Rather the information is extracted from either the model or from the lp or mps problem file. I updated the code examples at the top of the PR to explain how it works.

As far as code coverage is concerned, I have not yet been able to get around to tackling that with adjusted unit tests unfortunately. Apart from this, @lkstrp, I think it's ready for your review.